1. Field of the invention
The present invention relates to a device to limit inrush current, specifically, to a device to limit the inrush current which may arise in lamp on its starting.
2. Description of the prior art
The resistance of a cold filament is, generally, one-tenth of that at the incandescent state. Energization of the rated voltage permits a high inrush current into the filament possibly to the damage of the filament and/or power source.
I discovered that such inrush current can be effectively limited with a device wherein a controlled rectifier, having a main current path connected in parallel with an inrush current limiting resistor, is connected in series with the lamp, and wherein the conduction of the controlled rectifier is controlled with a delay circuit in such manner that the power source energizes the lamp through the resistor over the time as predetermined by the time constant of the delay circuit, and disclosed this in Japanese Patent Laid-Open Nos. 215,697/84, 215,696/84 and 230,298/84.
Since in conventional device the triggering voltage of the controlled rectifier extremely varies with the change in junction temperature, for example, from 0.9 to 0.6 volts in the temperature range of -40.degree. C. to +40.degree. C., the operation point of the controlled rectifier is dependent on the ambient temperature, and, at a relatively high ambient temperature, the destruction by overheating may shorten the life of the controlled rectifier.